How to design a discrete linear regulator with an opamp and an N-channel MOSFET?

Question

So as far as I understand, the basic idea of a linear regulator is to use a series-pass element along with an opamp with negative feedback to stabilize the output. This is what I usually see as an example:

^{simulate this circuit – Schematic created using CircuitLab}

Now, to use a MOSFET in place of the BJT, can I just do a direct replacement like this:

^{simulate this circuit}

Or should the gate of the MOSFET be driven by another BJT like below?

^{simulate this circuit}

Or does it not make any difference? Is one more stable than the other?

Also I remember reading somewhere on the internet that in order for the opamp to regulate correctly, a small current should flow out of the output through the negative feedback loop, but a MOSFET gate doesn't allow any current in, which is also confusing.

Answer 1

It is quite possible that none will work effectively without a compensation network applied directly to the op-amp. Whether it's an op-amp supplying a load directly or via a buffer transistor, adding bulk capacitance to the voltage output will shift the phase margin towards the point of instability: -

The object of the above modification is to reduce the gain of the op-amp at higher frequencies so that the excessive phase shift brought about by the bulk output capacitance won't act on a part of the spectrum where the loop-gain is greater than unity. You might have noticed that some regulators do state that they have a maximum capacitance that can be added to their output - this is the same issue - they have internal compensation that counters "so-much" bulk capacitance but no-more.

Regards driving the MOSFET, because it is a source follower, it's gate-source capacitance won't be fully seen at the op-amp output and it might be OK stability wise but the bulk output capacitance is still present and is still a problem (see above). There's no real reason to use circuit 3 given what I've said above and, if you did you would need to add an emitter resistor to ground.

Simulators are a good friend for fixing and uncovering the problems mentioned above.

Answer 2

On the face of it your first circuit will work .Your second circuit could work if there was a pulldown resister on the gate of the mosfet .Now if the mosfet is big it will load up the opamp with lots of capacitance affecting opamp stability .You will then have to work on the control loop.Also remember that there is no current limiting so if you do not add this the fet could blow up if the load was a short circuit.